A description of the genomic alterations associated with the loss of nearly every single non-essential gene contained in the yeast Saccharomyces cerevisiae gene-knockout collection (YKOC) is presented in Nature this week. To build the resource, researchers sequenced the whole genomes of nearly all of the 4,732 strains comprising the homozygous diploid YKOC, extracting information on copy-number variation of tandem and interspersed repetitive DNA elements.